Cold air coming from ceiling diffusers on a winter morning is one of the most common complaints from homeowners with positive pressure ventilation systems. At MiHT Home Energy Care, it's a question we hear regularly. DVS, SmartVent and HRV all draw from the same source: your roof cavity. In winter, that roof cavity is cold.
Here is what's happening, why it happens, and what your controller is actually doing about it.
Positive pressure ventilation systems pull air from your roof cavity, pass it through a filter, and push it down into your living areas through ceiling diffusers. As fresh air enters, slightly stale indoor air is pushed out through natural gaps around doors and windows.
In a New Zealand home, the roof cavity is unlined and unconditioned. On a warm summer day it can reach extreme temperatures. In the middle of winter, particularly overnight or on overcast days, the cavity temperature drops to near the outdoor air temperature.
The system doesn't generate heat. It moves air from where it is to where you are.
In summer, the roof cavity air tends to be warmer than the air inside your home. The system pushes warmer air in. In winter, the opposite is true. The system is still running, still moving air, but the air it's drawing from is cold.
The result is what many homeowners describe: diffusers pushing cool air into bedrooms on winter mornings, the house feeling harder to heat, and power bills that creep up without a clear reason.
The system is operating as designed. It moves air regardless of that air's temperature.
Each system handles cold roof air differently, and each has settings designed to reduce the problem. Most homeowners have never been shown these settings exist.
DVS has a Heat Retention setting. When enabled, it automatically reduces the fan speed when the roof air is colder than the air inside your home. DVS also has a Winter mode, which increases airflow automatically when the roof space rises above 19°C on a sunny day, capturing available solar warmth.
The Heat Retention setting ships from the factory switched off. Many DVS systems running in NZ homes right now are operating without it enabled.
SmartVent systems have a minimum temperature protection setting. The factory default is 5°C. When the roof cavity drops below that threshold, the controller turns the fan off or drops it to a very low speed. This setting is adjustable, and 8°C is a more practical threshold for most Auckland homes in winter.
HRV uses a ventilation level scale from 1 to 6. Higher settings move more air and clear condensation faster, but they also bring more cold air in. Lower settings reduce the cold air but move less moisture out of the house.
HRV's own guidance suggests levels between 3 and 6 in winter for condensation control. If cold air is your main issue, starting at level 3 rather than 5 or 6 is a practical balance. It provides enough airflow to manage moisture without driving cold air through the house at full volume. Adjust from there based on whether condensation is building.
Even at low settings, the HRV continues to run at a minimum ventilation level when the roof temperature drops below your set temperature. This keeps enough airflow going to prevent condensation, while causing at most a one-degree change in indoor temperature.
Not every system in the field has the same controller. Older units, and some of the systems installed in NZ homes are fifteen years old or more, may have simpler controls without automatic temperature thresholds. If you're unsure what your controller can do, your operating manual is the right place to start. DVS, SmartVent and HRV all make manuals available on their websites.
What every system can do, regardless of age or model, is have its fan speed adjusted manually.
If you know a cold night is coming, turning the fan speed down before bed is a practical step. Less speed means less cold air moving through the house overnight. During the day, when the roof cavity tends to warm up, particularly on any sunny winter day, raising the speed again gives the system time to clear moisture and dry the air out.
It isn't a perfect arrangement. But it makes real use of a system that's already installed, and any homeowner can do it without calling anyone.
Lowering your fan speed overnight, whether through your controller settings or manually, may mean waking up to condensation on windows. That's worth knowing before you make the change.
The reason is straightforward. Less airflow overnight means less moisture being pushed out of the house. Cold surfaces, particularly glass, collect that moisture as condensation.
What matters is what happens next. Once the system runs at a higher speed during the day and the roof cavity warms up, that condensation will clear. A house that sees some window condensation in the morning but is well ventilated through the day is in a better position than one where the system has been switched off entirely.
The aim is airflow when it's most useful, during the day, rather than cold air through the night when it's least comfortable and least welcome.
If your system is blowing cold air in winter, check your controller settings before switching it off.
DVS: Look for the Heat Retention setting in your controller menu. If it's switched off, your operating guide has the steps to enable it.
SmartVent: Check the minimum temperature setting. If cold air is still coming through below the current threshold, raising it to 8°C is a reasonable adjustment for most Auckland homes.
HRV: Check your ventilation level. If you're running at 5 or 6 through winter, dropping to 3 or 4 may reduce the cold air without losing meaningful condensation control.
Your operating manual has the exact steps for your model. SmartVent and DVS both have how-to videos on their websites if the manual isn't to hand.If you'd like someone to check the full system, including controller settings and condition, what a professional ventilation service includes covers what that visit should look like.
These systems draw air from your roof cavity and push it into your home through ceiling diffusers. In winter, your roof cavity drops to near outdoor temperatures, particularly overnight or on overcast days. The system moves air regardless of its temperature, so when the cavity is cold, cold air comes in. The fix is not to switch the system off but to check your controller settings. DVS systems have a Heat Retention setting, off by default, that automatically reduces fan speed when roof air is colder than the air inside. SmartVent systems have a minimum temperature protection setting, defaulting to 5°C, which slows or stops the fan below that threshold. Check your operating manual for your specific model.
Switching the system off is not recommended. A ventilation system running at a lower fan speed in winter is more useful than one that has been switched off. Without any airflow, moisture builds inside the house, particularly on windows and cold walls, and mould can establish over a winter. The better approach is to lower the fan speed, either through your controller's winter settings or manually, to reduce the cold air coming in while keeping enough airflow to manage moisture. During the day, when roof cavity temperatures recover, running at a higher speed clears any condensation that built up overnight. Check your controller settings before reaching for the off switch.
HRV recommends ventilation levels between 3 and 6 in winter for condensation control. Higher settings move more cold air into the house. If cold air is your main issue, starting at level 3 is a practical balance. It provides enough airflow to manage moisture without driving cold air through every room at full volume. Adjust from there. If condensation builds on windows overnight, raise the level slightly. If the house feels too cold in the morning, drop it back. The ventilation level is adjusted through the controller. Your operating manual has the steps for your model, and HRV's customer line on 0800 478 123 can walk you through it.
Yes. DVS systems have two winter-related settings. The Heat Retention setting automatically reduces fan speed when the roof air is colder than the air inside your home. The Winter mode increases fan speed automatically when the roof space rises above 19°C on a sunny winter day, capturing free solar warmth. The Heat Retention setting ships from the factory switched off, so many DVS systems in NZ homes are operating without it enabled. Check your controller menu for the Heat Retention setting and refer to your operating guide to enable it. Older DVS models have the same adjustment available through manual fan speed control.
If you have lowered your fan speed or your controller's winter settings have reduced airflow overnight, some morning condensation on windows is normal. Less airflow means less moisture is being pushed out of the house, and cold glass collects that moisture as condensation. This does not mean the system has failed. Once the system runs at a higher speed during the day and the roof cavity warms up, that condensation will clear. A house with some morning window condensation that clears during the day is being managed. If condensation is building and not clearing by midday, your fan speed may be set too low for your home's moisture levels.
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